Not saying you don't need the HD Pro, but most things including recording and home playing can be done with an HD500X (and cheaper ;-).

I think as long as you just record with an HD device (Pro or 500) the builtin USB is just fine.

If you think about recording multiple tracks at the same time combining a Pod with other sources you will probably need a suitable interface because many SW packages can really only handle 1 device for multi-track recording. But you can always add that later. I'd just carefully check whether the extra Pro features are really needed because the price dfference could pay for a fair bit of other things you may want. Enjoy